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How to calculate available disk space on a RAID 5

Storage management expert Ashley D'Costa answers a reader's question: "How do I calculate the disk space available on my RAID 5?"

I have three disks, each different sizes (60 GB, 80 GB and 40 GB) and I have configured RAID 5 on them. How do you calculate the disk space available for data storage and how much disk space is utilized for parity storage?

First off, I would not recommend building a RAID 5 group on non-like disk drives. Best practices dictate that all drives should have the same speeds and same capacities.

Unless you have a very special type of RAID 5 adapter, RAID 5 requires that capacity across your drives has to be the same. When you mix drive sizes, the drive with the smallest capacity is the total space you can utilize per drive. So, in your RAID 5 set comprised of 60 GB, 80 GB and 40 GB drives, the most you can get is 2 x 40 GB = 80 GB with the third 40 GB going to parity. All remaining space (60 GB) is wasted and totally inaccessible.

I recommend acquiring three 80 GB drives (or whatever size you need) and setting up a RAID 5 on those. The result will be 2 x 80 GB = 160 GB with 80 GB going to parity (Regardless of the number of drives in the set, one drive's worth of space is always lost to parity in a RAID 5 configuration).

This was first published in October 2007

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